Q: What does a rear diffuser extension actually do for the car?

It lengthens the factory diffuser and adds two central strakes that guide air more effectively through the rear tunnel. That extra working surface sits where pressure is lowest, which adds rear grip, and it smooths the transition as air exits into the wake, which is how a diffuser trims drag. The strakes also delay flow separation so air stays attached farther back. The result is a rear end that feels more planted at speed.